⚠️ This repository is no longer maintained. Please use the official Go http.MaxBytesReader and its example methods to limit upload file size, ensuring application security and performance. For Gin-specific usage, refer to Gin official example: limit-bytes.

Limit size of POST requests for Gin framework

When handling file uploads, it's crucial to check for middleware errors first to avoid response conflicts:

func fileUploadHandler(ctx *gin.Context) {
  // Always check middleware errors FIRST
  if len(ctx.Errors) > 0 {
    // Size limiter has already sent HTTP 413 response
    return
  }

  // Safe to process file upload
  if file, err := ctx.FormFile("file"); err != nil {
    ctx.JSON(http.StatusBadRequest, gin.H{
      "error": "Failed to process file: " + err.Error(),
    })
  } else {
    ctx.JSON(http.StatusOK, gin.H{
      "success":  true,
      "filename": file.Filename,
      "size":   file.Size,
    })
  }
}

func badHandler(ctx *gin.Context) {
  // Wrong: This will cause response conflicts
  if file, err := ctx.FormFile("file"); err != nil {
    ctx.JSON(http.StatusOK, gin.H{"msg": "fail: " + err.Error()})
  } else {
    ctx.JSON(http.StatusOK, gin.H{"msg": "ok: " + file.Filename})
  }
}

When a request exceeds the size limit, the middleware will:

1. Add an error to ctx.Errors
2. Set Connection: close header
3. Return HTTP 413 (Request Entity Too Large) status
4. Send JSON response: {"error":"request too large"}
5. Call ctx.Abort() to prevent further processing

• Always check ctx.Errors first in your handlers
• Use realistic size limits (consider multipart overhead)
• Handle HTTP 413 responses properly on the client side
• Test both normal and oversized file uploads

r.Use(limits.RequestSizeLimiter(1024))      // 1KB
r.Use(limits.RequestSizeLimiter(1024 * 1024))   // 1MB
r.Use(limits.RequestSizeLimiter(10 * 1024 * 1024)) // 10MB

If you're experiencing duplicate JSON responses like:

{"error":"request too large"}{"msg":"fail: HTTP request too large"}

This happens when your handler tries to send a response after the middleware has already sent one. Solution: Always check ctx.Errors first!

fetch("/upload", {
  method: "POST",
  body: formData,
})
  .then((response) => {
    if (response.status === 413) {
      alert("File too large, please select a smaller file");
      return;
    }
    return response.json();
  })
  .then((data) => {
    console.log("Upload successful:", data);
  });

See the _example/ directory for complete working examples:

• _example/main.go - File upload best practices